Rustlers are running a competition to win tickets to a Premier League, Serie A or La Liga game by answering a question on their site (the link should work & the answer is B - San Siro). After answering the question they will email you a voucher for buy one get one free.
It can be used on any of the Rustler range, however Sainsburys have the 3 sub versions (Tangy Meatball, Southern Fried Chicken & Chicken, Bacon & Cheese) for 99p each.
The voucher will be emailed as a PDF with no need to download any software, and it can be saved and re-used again & again. Not sure when the offer ends.